The present invention relates to an optical head device, and more particularly to an optical head device operable with optical disks of various sizes, such as compact disks of diverse thicknesses.
Generally, optical disk devices have used an optical head having a transparent substrate, through which a laser beam is transmitted to form a micro-spot on a micron-order surface of a storage medium, wherein the transparent substrate protects the micron-order surface of the storage medium. The transparent substrate is a transparent parallel plane plate through which a converging beam is transmitted, for which reason a wave-front aberration depending upon a thickness of the parallel plane substrate appears on the transmitting beam. If the wave-front aberration is large, then a convergence spot is enlarged beyond a diffraction limit whereby it is difficult to obtain desired recording/replaying properties. In order to settle those problems, it is required to form a micro-spot as small as possible in the light of the diffraction limit on the storage medium surface to compensate such wave-front aberration.
The regulation of the thickness of the optical disk is variable, depending on the use. If the thickness of the optical disk is varied, then the amount of compensation to the wave-front aberration is also varied, for which reason the conventional optical head is operable with recording/replaying optical disk mediums having a certain thickness and being designed under a certain regulation. Namely, the conventional optical head is not operable with the various optical disk mediums designed in accordance with the various regulations.
In order to settle the above problem, an optical head device was proposed, which is capable of switching twin lenses in accordance with the various optical disks wherein the twin lenses are mounted on an actuator. This optical head device is disclosed in the "TECHNICAL REPORT OF THE INSTITUTE OF ELECTRONICS INFORMATION AND COMMUNICATION ENGINEERS", MR95-25, August 1995.
FIG. 1 is illustrative of a mechanical structure of the actuator provided in the above conventional optical head device having twin lenses. The actuator is provided with a movable plate 10 on which a first objective lens 7 and a second objective lens 8 are mounted. The movable plate 10 is provided to rotate around an axis 21 in a predetermined angle range. The first and second objective lenses 7 and 8 are spaced from the axis 21 around which the movable plate 10 rotates so that the first and second objective lenses 7 and 8 are switched by the rotation of the movable plate 10. Either a reflective mirror or a reflective prism is provided on a base of the actuator so that the reflective mirror or the reflective prism is fixed in position under one of the first and second objective lenses 7 and 8. An incident beam 20 is reflected by the reflective mirror or the reflective prism. The movable plate 10 has a side edge on which a pole piece 22 is formed. A magnet 24 is slightly distanced from the pole piece to provide a magnetic force to the pole piece 22 whereby the movable plate 10 is forced to rotate around the axis 21. A tracking coil 23 is provided on the magnet 24 so that a large current is applied on the tracking coil 23 to rotate the movable plate 10 for switching the first and second objective lenses 7 and 8. Bistable points exist in the magnetic field between the magnet 24 and the pole piece 22 so that the movable plate 10 is stable in the bistable points of the magnetic field wherein the first and second objective lenses 7 and 8 are positioned over the reflective mirror or the reflective prism. The tracking is made around the axis 21, in the light of which the movable plate 10 is made to rotate at a larger angle than in the tracking process, so as to switch the first and second objective lenses 7 and 8. The first and second lenses 7 and 8 are adjusted to the different two optical disks having different thicknesses so that the above optical head is operable with two different optical disks having different thicknesses by switching the first and second objective lenses adjusted to these two different optical disks.
For the above conventional optical head, the slidable and rotable actuator which is allowed to rotate at a relatively large angle is usable. Notwithstanding, a wire-supporting actuator or a plate spring supporting actuator are not available due to those small movable ranges even those actuators have widely been used.
In the above circumstances, it had been required to develop a novel twin lens optical head device operable with two different optical disks having different thicknesses but not limiting the type of the actuator or allowing various types of the actuator.
It has been also required to develop a novel twin lens optical head device operable with two different optical disks having different thicknesses and capable of recording/replaying operations with a considerably reduced optical loss.